On PC it was a more modest 15GB, but players who have bandwidth caps that are already stretched to their limit by fall releases aren't all happy, with the game requiring nearly 100GB by itself in just a couple of weeks. The developers did mention this prior to releasing the update, and said "This update will be large compared to what we expect for patches going forward." Of course, Overwatch just reinstalled itself with a new patch for at least the second time, so maybe it's time for ISPs to ease up on bandwidth restrictions because the file sizes aren't going down.

Update: The size of the patch is similar between PS4 and Xbox One, leaving PC gamers with the smaller 15GB download.